One of the largest weapons in the arsenal of the insurance companies is withholding of medical care for the injured worker. If they deny your medical claim, or accept only an elbow injury (for example) and not your shoulder and/or back injury, your life line is to use your group insurance to pay for the other parts of your body that need medical care.
If you ultimately win your case, your group carrier will want their money paid on your behalf to be reimbursed.
Many group insurance companies will argue they have no obligation to pay because your complete condition is the responsibility of the workers' compensation insurance company.
You just have to show that the workers' compensation insurer has controverted your claim, or parts of it. I have had little problem getting medical treatment paid for by group insurance so long as my client and I agree to reimburse them when and if workers' compensation is found to be responsible for the medical treatment.
I try to have my client pay a very small amount of the co-pays or deductible to insure continuing medical treatment and pay the balance due at settlement of their claim. This may not always work but I find the group insurance companies a lot friendlier than the workers' compensation insurers.